This invention relates to electret filter media having enhanced charge stability.
Electret filter media have long been used in many filtration applications. Electret filter media are those that include a dielectric insulating polymer web that is treated to possess substantially permanent spatially oriented, opposite charge pairs or dipoles. Among the common polymer webs used for electret filter media are polypropylene, polyethylene, polyester, polyamide, polyvinyl chloride, and polymethyl methacrylate.
Conventional filter media are substantially lacking in electrostatic charge and rely upon impingement, impaction and diffusion for filter performance. Electret filter materials offer improved filtering performance over conventional filter materials. The presence of oriented dipoles in the electret filter media is believed to enhance filter performance by allowing the filter media to attract and retain charged and uncharged particles to be filtered.
Electret filter materials are made using a variety of known techniques. One technique for manufacturing electret filter media involves extruding a polymer, typically having a high melt flow index, through a die having a linear array of orifices. An air knife is used to attenuate the extruded polymer fibers by a ratio of about 300:1. The attenuated fibers, having diameters of about one to ten micrometers, are collected on a rotating drum or moving belt using a moderate vacuum. The fiber web is then treated to impart on the fiber web charge pairs or dipoles. The charge pairs or dipoles can be imparted to the fiber, for example, using AC and/or DC corona discharge.
One problem associated with electret filter material is that the charge pairs or dipoles imparted to the filter media often are not stable. In some instances, charge or its spatial orientation, is lost after filtering certain contaminants for relatively short time periods. The result is a marked decrease in filter performance over a relatively short period of time (e.g., less than 20 minutes). The National Institute of Safety and Health (NIOSH) has established standards for performance of certain filters. The NIOSH standards evaluate filters in a carrier after a 200 milligram filtration challenge. One challenge of solid aerosol particles evaluates filter performance against solid sodium chloride particles suspended in air. Another challenge of liquid aerosol particles evaluates filter performance against liquid droplets of dioctyl phthalate (DOP) suspended in air. Electret filter media generally are able to maintain charge stability and filter performance when filtering solid aerosols, including the sodium chloride test standard. However, liquid aerosols tend to degrade the charge on the electret filter media, and thus filter performance diminishes after only a short period of filtration.
Accordingly, there is a need for electret filter media having improved charge stability and that are able to maintain acceptable filter performance over time.
The present invention circumvents the problems described above by providing an electret filter media that retains particles and/or oil without significant reduction in filtration performance, even after prolonged filtration challenges. In general, the present invention is directed to a charge stabilized electret filter media that provides enhanced filtration performance characteristics. The invention provides filter media that comprises a hydrophobic and/or oleophobic vapor phase deposition treated electret polymer meltblown fiber web.
In one embodiment, a filter media is provided having formed thereon a polymeric coating that is created through a vapor phase deposition process. In another aspect, the invention relates to a method for manufacturing a filter media that comprises a meltblown hydrophobic and/or oleophobic vapor phase deposition treated electret polymer media.
The invention provides filter media having a meltblown hydrophobic and/or oleophobic vapor phase deposition treated electret polymer fiber web. Optionally, the meltblown web is deposited on a spunbond layer. The polymer web of meltblown polymer fibers is coated with a hydrophobic and/or oleophobic monomer by a vapor phase deposition process. The hydrophobic and/or oleophobic monomer is a suitable fluorine-containing monomer. Following deposition, the monomer is polymerized, such as by contact with a sufficient quantity of a suitable source of energy. The web is then treated to form substantially permanent charge pairs or dipoles therein. Permanent dipoles, e.g., electret characteristics, can be imparted to the web by a variety of techniques including AC corona or DC corona discharge and combinations thereof. In one embodiment, the manufacturing process can be modified by heat treating, e.g., annealing, the polymer web prior to vapor phase deposition.
In one embodiment it is possible to clean or prepare the surface of the meltblown web, prior to the vapor deposition step, by a technique such as plasma treatment, heat treatment, or flame treatment of the web.
The hydrophobic and/or oleophobic monomer useful for the vapor deposition process can be a hydrophobic and/or oleophobic material such as an alkylene, acrylate, methacrylate, alkyl oxirane, or alkylene oxirane. Preferably the monomer is halogenated, and most preferably it is fluorinated.
In one embodiment, the web may be a meltblown web that includes a charge stabilizing additive, such as a melt processable fatty acid amide. Typically, the charge stabilizing additive is present at a concentration in the range of about 0.01% to 20% by weight.
Although the fiber web is sometimes referred to herein as a meltblown web, it is understood that a variety of polymer and glass fiber webs may be used in accordance with the present invention. Suitable types of polymer webs include carding webs, spunbond webs, and spun laced webs.
The filter media of the invention may be used in a variety of filtration applications, including use in industrial face masks or respirators, indoor air quality filters, surgical masks, room air cleaners, cabin air filters, vacuum filters, HVAC filters, HEPA filters ASHRAE filters and ULPA filters. It is understood that the term xe2x80x9cfilterxe2x80x9d as used herein encompasses any device with which filter media of the invention may be used to filter air and/or other gases.
The electret filter media of this invention is characterized by improved filtration performance and enhanced charge stability of the electret polymer web. In particular, the filter media is able to provide desirable filtration properties, as indicated by alpha value, despite continued filtration challenge. In one embodiment, the filter media meets the NIOSH standard for class P 95 nonwoven filter media. In another embodiment, the filter media meets the NIOSH standard for class P 99 nonwoven filter media. Preferably, the filter media meets the NIOSH standard for class P 100 nonwoven filter media.
Other advantages of the invention will be readily apparent to one having ordinary skill in the art upon reading the following description.
All percentages by weight identified herein are based on the total weight of the web unless otherwise indicated.